National security affairs adviser William P. Clark has emerged as the strong man of President Reagan's many-sided policy in Central America, according to the assessment of administration officials close to the process.

These officials cite Clark's role in all of the important military and diplomatic decisions that have led to increased U.S. involvement in Central America. They point out that Clark was an advocate and implementer of the increase in covert aid to rebels in Nicaragua proposed by CIA Director William J. Casey and that he favored the large-scale military and naval exercises in Central America put forward by Pentagon planners.

On the diplomatic front, Clark is given the credit, or blame, for choosing former secretary of state Henry A. Kissinger as chairman of the National Bipartisan Commission on Central America.

Special envoy Richard B. Stone, who nominally reports to the State Department, also was Clark's choice, reportedly over the objection of Thomas O. Enders, the former assistant secretary of state for inter-American affairs. The choice of Stone put the White House stamp on negotiations which Stone has conducted with leaders of the Sandinista Nicaraguan government and the leftist guerrilla movement in El Salvador.

"On Central America, Clark is Reagan's personal representative on an issue of high concern to the president," is the way one administration official described it. "He's energizing the system."

Clark's pre-eminence in all aspects of the process has shattered his passion for anonymity and yesterday prompted the administration to play down his importance. After Clark appeared on the cover of Time magazine this week, White House officials made a public show of emphasizing the involvement in Central American policy of Secretary of State George P. Shultz, who yesterday gave a well-publicized briefing of Republican congressional leaders.

One source said that the attention given to Shultz, who will appear on Capitol Hill today, "will show that State isn't being shunted aside." But the public relations effort was, as one official put it, "so labored and obvious," that it may have had the opposite effect.

Clark's role is not that of a lone wolf, his boosters and critics in the administration agree. They say he has been effective because he is close to the president, whom he served as chief of staff in 1967 during Reagan's first term as governor of California, and because he understands how to push presidential decisions through the bureaucracy.

On the subject of the Kissinger commission and the Stone appointment, Clark worked closely with U.N. Ambassador Jeane J. Kirkpatrick, whom he sent on a fact-finding mission to Central America last February. She returned with recommendations for increased expenditures for economic and "humanitarian" aid, which have been incorporated into preliminary and unannounced budget proposals for next year and are expected to be a major focus of the Kissinger commission.

On the question of expanded aid to the U.S.-backed, anti-Sandinista forces in Nicaragua, Clark reportedly sided with Casey, whose proposals to expand the "contra" force to its current level of 12,000 troops was opposed by some senior CIA officials.

Clark did not directly propose the stepped-up level of military maneuvers, officials said. But he asked the question at a National Security Planning Group, saying the president wanted to know what could be done to "keep the pressure up" in Central America.

The result was a speed-up in the timetable and an increase in the size of the military and naval forces that will stage protracted exercises in the region.

In this case, officials said, the proposals were pushed, with Clark's backing, by Under Secretary of Defense Fred C. Ikle and and Deputy Assistant Secretary Nestor D. Sanchez.

Secretary of Defense Caspar W. Weinberger's role is a matter of some controversy in the administration. In 1981, Weinberger clashed with then-Secretary of State Alexander M. Haig Jr., who he saw as favoring the use of U.S. combat troops in the region.

Weinberger said in an interview with The Washington Post that his current support of administration policy is "fully consistent" with his earlier views since U.S. troops will not now be involved in any combat role. However, another high official in the administration referred approvingly to Weinberger as "a good soldier," a term usually reserved for officials who have opposed policy positions but are supporting them anyway.

This official, and others, also suggested that Shultz was far less centrally connected to the policy decisions than would be suggested by yesterday's briefing of GOP congressional leaders at the White House, in which he played the central role.

"Shultz wants to emphasize the diplomatic solutions, which go with his job," one official said. "But this is a combination of heavy pressure and diplomacy."

When Langhorne A. (Tony) Motley, then the ambassador to Brazil, was named to replace Enders as the assistant secretary of state for inter-American affairs last May with Clark's approval, it was expected that he would play a policy leadership role.

So far this expectation hasn't been met. Only last week, a press aide said that Motley didn't want to be interviewed because he didn't know that much about policy details. Yesterday, an official acknowledged that "Motley's not firmly in the saddle yet."

Whatever the reservations may be outside the White House, there is unity within, officials said. On many issues there have been pointed differences between Clark and White House chief of staff James A. Baker III, who pays close attention to Hill opposition to administration policies.

But on Central America, Baker's views are known to be as tough-minded as President Reagan's or as Clark's.

"He has seen to it that his subordinates are supportive on this issue, and that's very helpful," said one official close to the national security adviser.

This White House unity is seen by officials throughout the administration as strengthening Clark's role as the president pursues his many-side policy in Central America.